The accuracy and reliability of basic sediment and water (BS&W) measurements as made by the capacitance-type recorders are dependent upon the extent to which the intrinsic dielectric constant of the subject fluid (e.g. pipeline crude oil) varies with time. The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. Thus, if one or both of these properties should vary, the recorders measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because the recorders must be initially set to read zero BS&W at the intrinsic dielectric constant of the fluid. The capacitor-type recorders of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned.
Automatic compensation for erroneous BS&W measurements could be obtained if a clean, dry sample of the line fluid were continuously provided for measurement of its intrinsic dielectric constant. In this way the true BS&W content of the fluid could be measured by finding the difference between the dielectric constants of the wet and dry streams.
Automatic compensation has been delayed primarily because of the difficulty of designing an apparatus which would provide a continuous BS&W free sample from the line fluid. Some of the stringent requirements imposed upon the design and operation of such a unit are as follows: First, it must effect sufficient BS&W removal to yield a clean, dry effluent of less than approximately 0.01%V BS&W without altering the physical properties of the fluid. For this reason mechanical separation is mandatory; the use of demulsifiers is not acceptable. Second, it should be able to operate at normal line pressures, or at pressures higher than the bubble point of the most volatile component to eliminate cavitation. Third, it must be able to accommodate liquids having high as well as low viscosities, and liquids containing corrosive compounds. Fourth, it should be simple and economical in design and operation, virtually maintenance-free, and suitable for long-range and continuous service.
Providing a clean, dry, continuous reference stream without altering its physical composition is a difficult problem and the art has not been successful in this connection to date. Prior art considered pertinent to the present invention includes U.S. Pat. Nos. 3,546,926 and 3,189,180.